Question

Which of the following statements about enthalpy is incorrect?
Enthalpy and internal energy of a system are always identical
enthalpy is a state function
$\Delta H$ is the enthalpy change at constant pressure
Reactions which absorb heat have a positive DH

Answer 1

Answer on Question #82101, Chemistry/ Organic Chemistry

Which of the following statements about enthalpy is incorrect?

Enthalpy and internal energy of a system are always identical

Enthalpy is a state function

$\Delta H$ is the enthalpy change at constant pressure

Reactions which absorb heat have a positive $\Delta H$

Answer

First statement (Enthalpy and internal energy of a system are always identical) is incorrect.

Enthalpy and internal energy of a system are not always identical,

As $\Delta H_{\mathrm{rxn}} = \Delta U_{\mathrm{rxn}} + \mathrm{P}\Delta V_{\mathrm{rxn}}$

where $\Delta H$ is enthalpy change,

$\Delta U$ – change in internal energy of the system (reaction),

$\mathrm{P}\Delta \mathrm{V}$ – pressure-volume work.

Enthalpy and internal energy are equal in the case when reaction takes place at constant volume, where no work of expansion is done:

$$V = \text{const}, \Delta H_{\mathrm{rxn}} = \Delta U_{\mathrm{rxn}}$$

Answer provided by www.AssignmentExpert.com